Hand tools having two pivotally connected handles for moving jaws that a user operates with one hand are well known in the art. The basic tool has three parts: a unitary first member having a first handle on one end and a first jaw on the other, a unitary second member having a second handle on one end and a second jaw on the other, and an axle pivotally connecting the two together. Scissors and sheet metal snips are examples of such hand tools operated by one hand.
In many applications, the mechanical advantage offered by the simple leverage of three part hand tools is insufficient making it difficult or impossible to achieve the desired result with one hand. For example, cutting, crimping, and seaming sheet metal with a three part hand snipper, crimper, or seamer can require more hand pressure than the user can deliver, particularly when the cutting, crimping, or seaming must be performed over a protracted period of time.
One solution is to provide a compound lever action tool. FIGS. 1 and 2 illustrate side elevation views of such a prior art compound leverage hand tool 500 shown in the open and closed positions, respectively. Hand tool 500 is designed to be operated using one hand. Hand tool 500 has a first jaw member J1 and a second jaw member J2. The intermediate portions of jaw members J1 and J2 are pivotally connected at pivot P1. Hand tool 500 also has first handle member H1 and second handle member H2. One end of first handle member H1 is pivotally connected to an end of first jaw member J1 at pivot P2, and one end of second handle member H2 is pivotally connected to an end of second jaw member J2 a pivot P3. The intermediate portions of first handle member H1 and second handle member H2 are pivotally connected together at pivot P4. Pivots P2 and P3 are disposed between pivots P1 and pivot P4.
In the embodiment shown, first jaw member J1 and second jaw member J2 are shaped and dimensioned so that they combine to form a seamer, which is used to press material between the flat plates of first jaw member J1 and second jaw member J2. However, it may be appreciated that hand tools designed for other purposes such as cutting and crimping can also have the same compound leverage structural arrangement of pivots and levers. One such hand tool is the bolt cutter shown in U.S. Pat. No. 484,670 to H. K. Porter. It is noted in this tool that pivot P1 actually comprises two spread-apart pivots, yet the fundamental principle of the invention is the same as hand tool 500. It is further noted that hand tool 500 is designed to be used by one hand, whereas the tool of U.S. Pat. No. 484,670 is designed to be used by two hands.
FIG. 1 also shows the lever arms of hand tool 500. The tool provides compound leverage because it has two sets of levers. Lever arm A is the distance between the crotch of first jaw member J1 and pivot P1. Lever arm B is the distance between pivot P1 and pivot P2. Lever arm C is the distance between pivot P2 and pivot P4. Lever arm D is the distance between pivot P2 and the distal end of first handle member H1. Therefore the mechanical advantage MA of hand tool 500 is MA=(B/A)×(D/C). In the typical hand tool 500 shown, the length of the lever arms are about: A=0.5 inches, B=1.25 inches, C=0.8 inches, and D=5.25 inches. The mechanical advantage MA is therefore MA=(1.25/0.5)×(5.25/0.8)=about 16.4 which is generally referred to as 16.4 to 1.
Additional mechanical advantage can be achieved by adding an additional set of levers to provide the tool with double compound leverage. U.S. Pat. No. 1,348,735 also by H. K. Porter is an example of such a tool. The handles pivot together to provide leverage. The jaws pivot together to compound the leverage provided by the handles. Intermediate links pivot together to doubly compound the leverage provided by the handles. This tool is designed to be used by two hands.
Snippers, crimpers, and seamers having only simple leverage for use by one hand may be satisfactory for work on sheet metal and other applications where the material is very thin and the work to be done does not extend over a long period of time. Snippers, crimpers, and seamers having compound leverage for use by one hand such as tool 500 above may be satisfactory for work on sheet metal and other applications where the material is somewhat thicker. But there are other situations where snippers, crimpers, and seamers having double compound leverage for use by one hand would be advantageous both to work thicker materials and perform the work over longer periods of time and these are not available.